Treatment of OSA by CPAP flow generator systems involves the continuous delivery of air (or breathable gas) pressurised above atmospheric pressure to a patient's airways via a conduit and a mask. CPAP pressures of 4 cm H.sub.2 O to 22 cm H.sub.2 O are typically used for treatment of sleep disordered breathing due to OSA and/or central apnea, depending on patient requirements.
Treatment pressures for assisted ventilation can range up to 32 cm H.sub.2 O and beyond, depending on patient requirements.
For either the treatment of OSA or the application of assisted ventilation, the pressure of the gas delivered to patients can be constant level, bi-level (in synchronism with patient inspiration and expiration) or autosetting in level. Throughout this specification the reference to CPAP is intended to incorporate a reference to any one of, or combinations of, these forms of pressure delivery.
The prior art method for providing CPAP treatment includes a vent for gas washout of the gas flow. The vent is normally located at or near the mask or in the gas delivery conduit. The flow of gas through the vent is essential for removal of exhaled gases from the breathing circuit. Adequate gas washout is achieved by selecting a vent size and configuration that will allow a minimum safe gas flow at the lowest operating CPAP pressure, which, typically can be as low as, around 4 cm H.sub.2 O for adults and 2 cm H.sub.2 O in paediatric applications.
Existing vent configurations include single or multiple holes, foam diffusers, slots and combinations thereof. A reference herein to a vent may be understood to include a reference to one or more holes, foam diffusers, slots or any combination of them.
The flow of gas from the gas delivery system through the vent to atmosphere creates noise as the delivered gas, and upon expiration the patient expired gas including CO.sub.2, passes through the vent. Increasing CPAP pressure results in more gas passing through the vent which in turn creates more noise. Existing prior art vents produce excessive noise when CPAP pressures are raised above about 4 cm H.sub.2 O. This noise can adversely affect patient and bed-partner comfort. Existing vents are also inefficient as they allow more gas through the vent than is required for adequate exhaust gas washout and thereby require the flow generator to provide more flow than is necessary in order to maintain the required treatment pressure. Further, where treatment gas is being supplied, such as oxygen, surplus treatment gas is vented and thereby wasted unnecessarily. A similar waste occurs where the supplied gas is humidified.
It is an object of the present invention to overcome or at least ameliorate one or more of these deficiencies of the prior art.